I would think one of the specialty produce purveyors at the wholesale produce market at Olympic and Alameda should be carrying it. There are large operations like Melissa's, as well as specialty family run produce shops that sell primarily to markets and the restaurant trade. Its easy enough to drive in and walk around and ask who sells the item in question.

At Bristol Farms today I spotted potimarron squash (labelled red kuri, as noted here and elsewhere on the Web). So I got one and look forward to auditioning it as a possible side dish for the Thanksgiving table this year. Thanks to Professor Salt for bringing potimarron/red kuri/Hokkaido to our attention.